1. Field of the Invention
The present invention relates to a method for accessing a record medium, and more particularly to an improvement in access control in a rotating storage device used as an external storage device of a computer system which uses a rotating record medium having a number of information recording tracks.
2. Description of the Prior Art
In writing data in a magnetic disc device, a plurality of data blocks are written on the magnetic disc and the data blocks are read immediately thereafter to check if the data has been correctly written. If an error data block is detected by the read-after-write check, a magnetic head is positioned to an alternate block position in order to write a correct data block to replace the error data block. In the magnetic disc, the alternate block is located in an alternate track which is different from a group of tracks in which normal data blocks are located. Accordingly, an significant time is taken for the movement of the magnetic head from the normal track in which the error clock is located to the track in which the alternate block is located. After the data has been written in the alternate block and the correct writing has been confirmed by the read-after-write check, the magnetic head is again positioned at the track in which the error block is located and the read-after-write check is resumed from the data block next to the error block.
The reason for concentrating the alternate blocks in the particular track in the magnetic disc device in spite of a long access time required is that the magnetic disc device has a very high reliability and the occurrence of an error block is very rare.
On the other hand, an optical disc device has recently been used as a large capacity external storage device of the computer system. In the optical disc device, a laser beam modulated with data is directed to a metal film formed on a disc surface to form apertures called pits to write the data, and in reading the data, a weak laser beam is directed to the metal film and the presence or absence of the pits is sensed by a change in reflected light intensity. In the optical disc, the formation of the pits is subject to variation by the deterioration of a laser light source, focusing technology of the laser beam to the metal film and a condition of the metal film of the record medium. Accordingly, the probability of error in writing the data is higher than that of the magnetic disc. Thus, if the accessing method utilized in the prior art magnetic disc is employed in the optical disc device, the relocation of the head to the alternate block by the occurrence of a write error frequently occurs and the data processing efficiency is materially reduced.